Pervious concrete is one of the hottest topics in land development today. As owners, architects, land developers, and concrete professionals become familiar with its benefits, the interest in pervious concrete continues to grow. The use of pervious concrete pavements provides a solution to new requirements under Environmental Protection Agency regulations that call for decreasing the amount of surface water runoff and initially treating the runoff.

Pervious concrete is made of cementitious materials, water, admixtures, and narrowly graded coarse aggregate. Very little or no fine aggregate is used in the mixture. With just enough cement paste to coat the aggregate, a system of interconnected voids (15 to 35 percent) is created resulting in a highly permeable concrete that drains very quickly. By allowing water to pass directly through the concrete, the amount of surface water runoff is reduced dramatically. It can also be used as part of a system to reduce the level of pollution contained in storm water that is captured in the pervious pavement.

Pervious pavements have been used for years throughout the warmer climates of the United States with excellent results. However, in climates prone to severe freeze-thaw cycles, some are hesitant to use pervious concrete pavements until it has been proven that pervious concrete can be made to resist freeze-thaw damage. Resistance of any concrete to freezing and thawing depends on the permeability, the degree of saturation, the amount of freezable water, the rate of freezing, and the average maximum distance from any point in the paste to a free surface where ice can form safely. The rate of freezing in most applications is dictated by the local climate.

Entrained air may help protect the paste as well. Perhaps the most important aspect in designing pervious concrete pavements for freeze-thaw areas is avoiding, or at least limiting, saturation, especially during the time of year when freezing can be expected. It is possible to design pervious concrete pavements to control the degree of saturation and the average maximum distance to a free surface. Proper subbase design and preparation are keys to pulling rainwater, ice, and snowmelt away from the pavement and ensuring suitable drainage. Replacing as little as 7 percent of the coarse aggregate with fine aggregate increases the freeze-thaw resistance; however, there will be a reduction in voids of a few percent (Kevern 2006 and Mata 2008).

In addition, the paste (or mortar) should be protected by using air-entraining admixtures to create a sufficient air-void system. Kevern, Wang, and Schaefer (2008) found that the coarse aggregate properties play a large role in providing freeze-thaw durability, with absorption values below 2.5 percent being of greatest impact. The National Ready Mixed Concrete Association (NRMCA 2004) has developed guidelines for using pervious concrete in areas prone to freeze-thaw conditions.

NRMCA Recommendations

Dry Freeze and Hard Dry Freeze Dry freeze areas are those parts of the country that undergo a number of freeze-thaw cycles (15+) annually in which there is little precipitation during the winter. If the ground stays frozen as a result of a long continuous period of average daily temperatures below freezing, the area is referred to as hard dry freeze area. Since pervious concrete is unlikely to be fully saturated in this environment, no special precaution is necessary for successful performance of pervious concrete. However, a 4- to 8-inch thick layer of clean aggregate base below the pervious concrete is recommended as an additional storage for the water. Many parts of the western United States at higher elevations come under this category.

Wet Freeze

This includes areas of the country that undergo a number of freeze-thaw cycles annually (15+) and there is precipitation during the winter. Since the ground does not stay frozen for long periods, it is unlikely that the pervious concrete will be fully saturated. No special precaution is necessary for successful performance of pervious concrete, but a 4- to 8-inch thick layer of clean aggregate base below the pervious concrete is recommended. The middle part of the eastern United States falls under this category.

Hard Wet Freeze

Certain wet freeze areas where the ground stays frozen as a result of a long continuous period of average daily temperatures below freezing are referred to as hard wet freeze areas. These areas may have situations where the pervious concrete becomes fully saturated because frozen soil will have very low water permeability. The frost penetration depth (depth at which the temperature is at 32 degrees Fahrenheit varies throughout the country.

To design the pervious concrete pavement for freeze-thaw resistance the following is suggested by NRMCA :

1. Calculate the frost penetration depth in your area. In the Washington, D.C., area, for example, it is about 30 inches.

2. Calculate 65 percent of that. The Federal Aviation Administration (FAA) says that the top 65 percent should contain non-frost-susceptible materials and the bottom 35 percent may be in frost susceptible sub grade. It should be noted that the FAA uses the 65 percent limitation to prevent frost heave. In this case, the key factor is water infiltration. This is about 19.5 inches for the 30 inches frost penetration depth.

3. Provide pervious concrete pavement plus aggregate base equal to the number calculated. For a 19.5-inch calculation, a 6-inch thick pervious concrete pavement over a 13.5-inch thick aggregate base would be sufficient. The aggregate base must consist of clean well draining open graded aggregate base with less than 1.5 percent finer than 0.5 mm. If the frost depth is very high, for example 100 inches in North Dakota, additional measures can be taken to reduce the chances of a fully saturated pervious concrete pavement. A perforated PVC pipe can be placed in the aggregate base to capture all the water and let it drain. Pervious concrete in a freeze-thaw environment should always be air-entrained to provide additional protection.

High Ground Water Table

Pervious concrete is not recommended in freeze-thaw environments where the ground water table rises to a level less than 3 feet from the top of the surface or where substantial moisture can flow from higher ground.

Deicing

Chemical Usage Deicing chemicals used to maintain an ice-free, safe pavement surfaces for dense pavements may be used on pervious pavements; however, in many cases deicers may not be required to maintain a slip resistant surface. Snow fall followed by thawing temperatures allows snow melt to pass through the pavement so rapidly that liquid water is not available at the pavement surface to be refrozen as an ice coating. Ice-free pavement surfaces aid safety for pedestrians and vehicles. With appropriate plowing and limited use of deicers, moisture is removed from the pavement surface, again preventing moisture from freezing at the surface and causing icy conditions. As any melting from the use of deicers occurs, the melt passes downward into the pavement and in many cases leaves behind some un-dissolved deicer making it available to future snow and ice events. Dosages of deicing chemicals have been reported at reduced rates up to 70 percent (Houle 2008). This leads to reduced use of deicing chemicals and clear safe pavement surfaces with minimum cost for winter maintenance.

Summary

Colored pervious concrete walkway Example application of pervious concrete: colored pervious concrete walkway in Bainbridege Island, WA. (IMG15586) The benefits of pervious concrete pavements are well known, but concerns over the freeze-thaw resistance may prevent many designers from using pervious concrete in colder climates. There have been several pervious concrete pavement projects in dry and wet freeze areas demonstrating good field performance over several years. Research on freeze-thaw resistance of pervious concrete pavement is ongoing across the United States. Pervious pavements should be placed by experienced installers and the structure and surrounding details should be designed to accommodate the anticipated water flow and drainage requirements.

References

Freeze Thaw Resistance of Pervious Concrete(2004) National Ready Mixed Concrete Association, 17 pagesThere have been several pervious concrete pavement projects in dry and wet freeze areas demonstrating good field performance over several years. Recommendations for successful performance of pervious concrete pavements under the various freeze-thaw conditions have been provided. There is limited experience of performance of pervious concrete pavements in hard wet freeze areas. Therefore, in such areas utmost care must be taken. Pervious pavements should be placed by an experienced installer and the pavement structure and surrounding details should be designed to accommodate the anticipated water flow and drainage requirements.